OBJECTIVE: To describe and answer questions concerning the spread of contrast in patients receiving correctly placed lumbar epidural steroid injections (ESIs) under fluoroscopy. DESIGN: Prospective observational study. SETTING: An outpatient surgery center. PARTICIPANTS: Consecutive patients (N=114) receiving ESIs under fluoroscopy who met inclusion criteria. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Spread of contrast in relation to variables, including unilateral versus bilateral, anterior versus posterior, and spread more than 1 level caudally versus less than 1 level. Variables were examined in relation to needle tip placement, level of injection, and male versus female patients. All data were collected with fluoroscopy images in lateral and anteroposterior views after injection of 5 mL of fluid. RESULTS: Spread was greater than 1 segment caudally more than 75% of the time under all variables. Anterior versus posterior epidural spread on the lateral view was approximately even over all cases and anterior spread was found more often when the needle was within the width of the distal spinous process tip. Needle placement medial or lateral to the spinous process width also significantly affected whether the spread was unilateral versus bilateral. If the needle tip was lateral to the width of the spinous process, spread was unilateral 75% of the time, versus 45% of the time if the needle tip was medial. CONCLUSIONS: Contrast spread is affected by needle placement, with other variables kept equal, in the performance of an interlaminar lumbar ESI. These data support the performance of interlaminar lumbar ESIs with fluoroscopic guidance and provide some parameters with which to guide the injectionist.